Hybrid integrated optical system manufacture uses a number of techniques to install the optical components on an optical bench or submount. Generally, the techniques must be applicable to solder bonding the usually small, i.e., few millimeters square and smaller, and typically delicate optical components.
One technique includes templating. Optical component slots are formed in a metal or graphite template substrate. The template is placed over the optical bench and the optical components are then installed on the bench for subsequent solder attachment. Generally, templating is most applicable to the installation of optical components requiring a relatively low alignment precision. For example, it is commonly used to attach components, such as thermistors, to the optical bench top.
Another common approach relies on precision pick-and-place machines, which are typically variants of flip chip bonders. They can place optical components to precisions of between 2 and 10 micrometers and then solder attach the optical components in place.
The problem with using pick-and-place machines to assemble the optical systems is that the placement precision is still not that required for many micro-optical systems. Moreover, the bonders are expensive instruments, and the assembly speed is relatively slow since the installation and bonding of the optical components occurs in a serial fashion.
The present invention concerns an optical system assembly technique that is compatible with a batch assembly process. Specifically, at least the bonding processes for the optical components can be performed in parallel, rather than serially as in most bonders, to thereby decrease the per unit assembly times.
In general, according to one aspect, the present invention features a templating system that is applicable to optical systems constructed from bonded optical benches and optical components. Specifically, the template system comprises a template substrate that is placed over the optical bench. The substrate has at least one alignment slot that is formed through the substrate. This alignment slot has an alignment feature against which an optical component is registered.
According to the invention, in order to improve the accuracy of the alignment of the optical component on the optical bench, the slot has a reentrant sidewall extending from the alignment feature into the substrate. This way, there is a single point or near single point of contact between the optical component and the template, to thereby improve the placement precision for the optical component on the optical bench.
Moreover, when the slot is formed using photolithography-based etch processes, the position of the feature can be specified with a high degree of precision, especially when it is located on the side of the substrate adjacent to the patterning resist. A combination of anisotropic and isotropic etches can be utilized to form slope of the reentrant sidewall. In another implementation, a modified deep reactive ion etch (DRIE), or Bosch, process is used where the periodicity of sidewall passivation is varied to obtain the reentrant sidewalls.
In the current implementation, the optical component sometimes comprises an optical element such as a lens, filter, mirror, or a microelectromechanical system (MEMS) device in combination with a mounting structure. In one example, this mounting structure has at least one armature, possibly including a discrete flexure, enabling plastic deformation of the alignment structure to align the optical element relative to the optical system.
In the preferred embodiment, the optical components are metal bonded to the bench. Specifically, metal bond pads are preferably deposited on the bench. Bond pads typically comprise a solder alloy, typically including gold. This yields a mechanically robust connection.
Further, according to the preferred embodiment, an electrical via is provided in the substrate. This is commonly required when the substrate is not subsequently removed from the optical bench prior to testing or final deployment in its intended application. The electrical via provides access to electrical traces on the bench.
In one implementation, the electrical via is simply a hole in the substrate that is aligned over wire bond pads in the optical bench. In an alternative implementation, the electrical via comprises a conductive path either through the substrate, in the form of an inter-metalization layer titanium plug as used in the conductor chips, for example, or electrical traces that are deposited on the outer surface of the substrate.
In general, according to another aspect, the invention features a process for fabricating an optical system. This process comprises depositing a bond pad on the optical bench. The deposition can either be performed using photolithographic/liftoff processes or by solder preform placement. Further, a substrate is installed over the optical bench. Alignment slots are formed in the substrate.
The order in which the bond pad deposition, substrate installation, and alignment slot formation steps are performed depends on the implementation. For example, in one implementation, the installation step is performed before slot formation. The bond pads can be deposited either before the substrate installation or after the slot formation. In contrast, in another implementation, the substrate installation step is first performed after the alignment slot formation. Bond pad deposition can occur either before the installation step or after the slot formation step.
In any event, the alignment slots are formed with an alignment feature that projects into the slot from the slot""s sidewall. The optical component is then inserted into the slot and onto the bond pad. The optical component is registered against the alignment feature of the substrate.
Of note is the fact that the present implementation is compatible with wafer-level integration. Specifically, a wafer of substrates is bonded to a wafer of optical benches, using fusion bonding or metal bonding (solder, thermocompression, or ultrasonic), for example. Optical components can also be installed at the wafer level. Only after the optical components have been installed and bonded to the bench is the optical bench wafer singulated into individual optical benches.
The above and other features of the invention including various novel details of construction and combinations of parts, and other advantages, will now be more particularly described with reference to the accompanying drawings and pointed out in the claims. It will be understood that the particular method and device embodying the invention are shown by way of illustration and not as a limitation of the invention. The principles and features of this invention may be employed in various and numerous embodiments without departing from the scope of the invention.